If you want to apply it to multiple cells, first select all the cells that you want to apply it to, then enter the formula as it applies to the FIRST cell in your selection.
Excel will adjust it for the other cells, if you use relative/absolute referencing correctly (which you did not). You want to lock the column reference ("C"), but not the row reference, so the row references can "float".
So if you selected cells B2:B100, then formula you would use is (note there is no dollar sign in front of the row reference):
=$C2="delete"